1. Field of the Invention
The present invention relates to a noise suppression method for a wavelength division multiplexing transmission system with repeaters which have multistage optical amplifiers.
2. Related Background Art
In an optical communication system used for long distance communication on an optical fiber path trunk line system, it is common to use optical amplifiers located at a regular interval on an optical path in order to compensate for deterioration of the signal level, consequently the deterioration of the signal level is reduced.
In the past, an optical amplifier transmission system having multistage optical amplifiers using a single wavelength is known. In the single wavelength transmission of this kind, based on the characteristics of the length of the transmission system, the repeater and the optical fiber of the transmission system, an optimum design can be accomplished and an optimum transmission condition can be obtained.
Recently, the wavelength division multiplexing transmission system has been developed to increase the capacity of the system by using a plurality of wavelengths. In such a plural wavelength division multiplexing transmission system, it is necessary to design a system based on a different design concept from that of the single wavelength transmission system.
FIG. 8(a) shows an example of the configuration of the transmission path of a wavelength division multiplexing transmission system with repeaters which have multistage optical amplifiers. As shown in this figure, the dispersion shift optical fiber (DSF) 101-1, the erbium-doped fiber amplifier (EDFA) 102-1, the DSF 101-2, the EDFA 102-2, the cut-off shift fiber (EF) 103-1, . . . are serially connected in the transmission path.
The dispersion shift optical fibers (DSF) 101-1.about.101-k constitute most of the transmission path. The erbium-doped fiber amplifiers (EDFA) 102-1.about.102-n are optical amplifiers which compensate the signal power. The cut-off shift fibers (EF) 103-1.about.103-m whose zero dispersion wavelength is 1.3 .mu.m compensate for accumulated chromatic dispersion shift fibers (DSF) 101-1.about.101-k whose zero wavelength dispersion is 1.55 .mu.m.
Wavelength dispersion is a phenomenon based on the fact that refractive index varies for the different signal wavelengths even in a homogeneous medium, and therefore propagation velocity varies with wavelength. For instance, if wavelength dispersion occurs in an optical fiber in which a pulse-like optical signal propagates, the pulse shape begins to deteriorate, the signal spectrum broadens as the signal propagates through the optical fiber, and consequently intersymbol interference may occur.
In order to prevent this phenomenon, the cut-off shift fibers (EF) 103-1.about.103-m are inserted as equalization fibers at predetermined intervals. The chromatic dispersion values of these EF-103-1.about.103-m are made to have an opposite sign to that of the chromatic dispersion values of the DSF 101-1.about.101-k.
FIG. 8(b) shows that the accumulated chromatic dispersion value gradually decreases in a negative direction, as the optical signal propagates through DSF 101-1, 101-2, . . . . When the optical signal propagates through the EF 103-1 portions of the path whose chromatic dispersion values have an opposite sign, the accumulated chromatic dispersion value reaches zero whereby the chromatic dispersion characteristic is compensated. Therefore the accumulated chromatic dispersion characteristics of the transmission path shown in FIG. 8(a) appears periodically. More specifically this accumulated chromatic dispersion characteristic occurs in an optical fiber transmission system comprising 184 optical amplifier repeaters in a 6,000 km long-haul optical submarine cable in which the optical repeater span is about 33 km, the output of each repeater is +3 dBm, and the gain of each repeater is 7 dB.
A peculiar problem on the FIG. 8(a) wavelength division multiplexing transmission system is the mutual interference (cross talk) between the signal wavelengths, because this wavelength division multiplexing transmission system has a plurality of signal wavelengths; this mutual interference deteriorates the transmission characteristics. More particularly, four-wave mixing occurred in the optical fiber causing an increase in noise which deteriorates the performance of the transmission system.
These various kinds of noise cause increasing problems as the transmission distance becomes longer, and the prior optical transmission system is unable to achieve a good transmission condition because of the influence of various kinds of noises which accumulate on the path of transmission.
This degree of interference is affected by almost all design parameters such as the distance between the wavelengths, the dispersion characteristics of the optical fiber, the nonlinear optical characteristics, the characteristics of the output of the optical amplifier repeater, the system length and so on. Therefore in order to optimize the transmission characteristics of a wavelength division multiplexing transmission system, it is necessary to adjust or modify these factors (parameters).
It is especially necessary, when replacing a single wavelength transmission system with a wavelength division multiplexing transmission system, to readjust or to change the parameters mentioned above in order to improve the transmission performance.
For instance, FIG. 9 shows the spectrum of two original signals of a conventional wavelength division multiplexing transmission system, and FIG. 10 shows the spectrum of the transmitted signals. As shown in FIG. 9, the optical signal 1 and optical signal 2 which have different wavelengths are multiplexed, this wavelength division multiplexed signal propagates through the optical path, and the spectrum at the receiving side becomes the one shown in FIG. 10 because of the reasons mentioned above.
As shown in FIG. 10, noises spread the spectra of the optical signal 1 and optical signal 2, consequently the signal-to-noise ratio (S/N) after the transmission is, as shown in the table of FIG. 7, 9.9 dB for the optical signal 1 (CH1) and 3.8 dB for optical signal 2 (CH2) which is a deteriorated S/N. And a Bit Error Rate (BER), as shown in the table of FIG. 7, is 2.6.times.10.sup.-7 for the optical signal 1 (CH1) and 5.3.times.10.sup.-4 for the optical signal 2 (CH2) which is a rather greater Bit Error Rate.
The wavelength division multiplexing transmission system uses a plurality of wavelengths which are different from that of the optical signal designed for the single wavelength transmission, so that the transmission performance is deteriorated as mentioned above. Consequently the parameters must be changed to meet desired optical transmission conditions.
Specifically in an optical amplified repeater transmission system placed on the bottom of the sea, which has multistage optical amplifiers, the transmission band width of submarine facilities is limited, and the wavelength spacing between the optical signals are designed in accordance with the degree of the multiplexity of the optical waves.
But it is physically and economically difficult to readjust system components caused by changing a transmission system. For instance in a submarine cable system which has already been installed, there is a possibility that adjustment of the characteristics of the amplifiers and replacement of erbium-doped fibers etc. and optical fibers may be necessary, so it becomes necessary to recover the optical amplified repeaters and to effect replacement of new optical fiber submarine cables. This is as difficult or more difficult than replacing the existing system with a new submarine cable system.